High efficiency solar cells fabricated by inexpensive pecvd

ABSTRACT

A method for forming a photovoltaic device includes depositing one or more layers of a photovoltaic stack on a substrate by employing a high deposition rate plasma enhanced chemical vapor deposition (HDR PECVD) process. Contacts are formed on the photovoltaic stack to provide a photovoltaic cell. Annealing is performed on the photovoltaic cell at a temperature and duration configured to improve overall performance.

BACKGROUND

1. Technical Field

The present invention relates to photovoltaic devices and methods forfabrication, and more particularly to systems, devices and fabricationmethods that improve efficiency for devices fabricated with inexpensivedeposition tools.

2. Description of the Related Art

With the push for more efficient and cheaper solar energy, higherthroughput and lower expense tools are needed for producing solarpanels. Conventional solar panel production typically relies of plasmaenhanced chemical vapor deposition tools that require ultra high vacuumand low deposition rates (e.g., 1 angstrom/sec) in order to achievesufficient quality. Such processes are marked by low plasma power (toreduce cost and protect the device being fabricated) and low gaspressure. These processes result in low productivity and stand in theway of the impetus to further solar power as a long term energysolution.

SUMMARY

A method for forming a photovoltaic device includes depositing one ormore layers of a photovoltaic stack on a substrate by employing a highdeposition rate plasma enhanced chemical vapor deposition (HDR PECVD)process. Contacts are formed on the photovoltaic stack to provide aphotovoltaic cell. Annealing is performed on the photovoltaic cell at atemperature and duration configured to improve overall performance.

Another method for forming a photovoltaic device includes depositing ap-type layer on a transparent electrode of a transparent substrate;depositing an intrinsic layer on the p-type layer; depositing an n-typelayer on intrinsic layer, wherein at least the p-type layer, theintrinsic layer and the n-type layer are deposited by employing a highdeposition rate plasma enhanced chemical vapor deposition (HDR PECVD)process to form a photovoltaic stack on the substrate; forming acontacts on the n-type layer photovoltaic stack to provide aphotovoltaic cell; and annealing the photovoltaic cell at a temperatureof between about 155 and 250 degrees Celsius to improve overallperformance.

Yet another method for forming a photovoltaic device includes depositinga buffer layer on a transparent electrode of a transparent substrate;depositing a p-type layer on the buffer layer; depositing an intrinsiclayer on the p-type layer; depositing an n-type layer on the intrinsiclayer, wherein at least the p-type layer, the buffer layer, theintrinsic layer and the n-type layer are deposited by employing a highdeposition rate plasma enhanced chemical vapor deposition (HDR PECVD)process to form a photovoltaic stack on the substrate, the HDR PECVDprocess including a deposition rate of 3 angstroms per second or greaterat a vacuum pressure of between about 1 mTorr and about 90 mTorr;forming a contact on the n-type layer photovoltaic stack to provide aphotovoltaic cell; and annealing the photovoltaic cell at a temperatureof between about 155 and 250 degrees Celsius for a duration of betweenabout 5 minutes and about 10 minutes to improve overall performance.

A photovoltaic device includes one or more layers of a photovoltaicstack formed on a substrate by employing a high deposition rate plasmaenhanced chemical vapor deposition (HDR PECVD) process. Contacts areformed on the photovoltaic stack to provide a photovoltaic cell. Reduceddefect zones are disposed adjacent to contact regions in portions of thephotovoltaic cell and are formed by an anneal configured to improveoverall performance. The anneal included a temperature of between about155 degrees Celsius and about 250 degrees Celsius to render the reduceddefect zones more conductive.

Another photovoltaic device includes a p-type layer formed on atransparent electrode of a transparent substrate, an intrinsic layerformed on the p-type layer, and an n-type layer formed on intrinsiclayer, wherein at least the p-type layer, the intrinsic layer and then-type layer are formed by a high deposition rate plasma enhancedchemical vapor deposition (HDR PECVD) process. A contact is formed onthe n-type layer. Reduced defect zones are disposed adjacent to thecontact in the n-type layer and adjacent to the transparent electrode inthe p-type layer and formed by an anneal configured to improve overallperformance. The anneal includes a temperature of between about 155degrees Celsius and about 250 degrees Celsius.

Yet another photovoltaic device includes a buffer layer formed on atransparent electrode of a transparent substrate, a p-type layer formedon the buffer layer, an intrinsic layer formed on the p-type layer andan n-type layer formed on the intrinsic layer, wherein at least thep-type layer, the buffer layer, the intrinsic layer and the n-type layerinclude characteristics of a high deposition rate plasma enhancedchemical vapor deposition (HDR PECVD) process to form a photovoltaicstack on the substrate. The HDR PECVD process includes a deposition rateof 3 angstroms per second or greater at a vacuum pressure of betweenabout 1 mTorr and about 90 mTorr. A contact is formed on the n-typelayer of the photovoltaic stack to provide a photovoltaic cell. Reduceddefect zones are disposed adjacent to the contact in the n-type layerand adjacent to the transparent electrode in the buffer layer and formedby an anneal configured to improve overall performance. The annealincludes a temperature of between about 155 degrees Celsius and about250 degrees Celsius and a duration of between about 5 minutes and about10 minutes.

These and other features and advantages will become apparent from thefollowing detailed description of illustrative embodiments thereof,which is to be read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

The disclosure will provide details in the following description ofpreferred embodiments with reference to the following figures wherein:

FIG. 1 is a cross-sectional view of a photovoltaic device with reduceddefect zones formed in accordance with the present principles;

FIG. 2 depicts a graph showing current density (mA/cm²) versus voltage(V) for depicting photovoltaic device performance with and without ananneal; and

FIG. 3 is a block/flow diagram showing methods for fabricating aphotovoltaic device with a high deposition rate plasma enhanced chemicalvapor deposition (HDR PECVD) process and an anneal in accordance withillustrative embodiments.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In accordance with the present principles, methods and devices arepresented that provide improved efficiency to photovoltaic cellsfabricated with high deposition rate and high throughput processes.Layers for semiconductor devices are often deposited using lowdeposition energies. Low deposition energies are favored bymanufacturing since the low energy permits high quality a-Si:H layerdeposition and less energy usage. Designers also prefer low depositionenergies since the low energies create plasma with less energybombardment on a surface resulting in less damage to existing structureson semiconductor devices. In thin film Si solar cell applications, highdeposition energies are desired for depositing a p+ layer since thehigher energies increase the likelihood that a microcrystalline phasewill form. The microcrystalline phase can reduce barrier heights betweenthe p+ layer and an electrode (e.g., formed from a transparentconductive oxide (TCO)).

For example, photovoltaic stacks and, in particular, p-i-n structures (ap-type layer, an intrinsic layer, an n-type layer) or n-i-p structures(an n-type layer, an intrinsic layer, a p-type layer) include layerswhere a band offset at an interface can be improved based upon thecrystallinity of the layers—the higher power, the higher thecrystallinity. Higher crystallinity permits for tunneling currentthrough the barrier. In useful examples, such improvements can be gainedat an interface between a transparent conductive oxide (TCO) to the p+layer. A band offset at the TCO/p+ layer is unavoidable since alldeveloped TCO films are n-type.

In particularly useful embodiments, photovoltaic devices are constructedusing low cost deposition processes. These processes include a plasmaenhanced chemical vapor deposition (PECVD) (hereinafter referred to as“high deposition rate PECVD” or HDR PECVD) that employ a high depositionrate and use a rough vacuum. The HDR PECVD processes increase throughputbut may result in lower performance. To improve cell performance, thepresent inventors have discovered methods for improving the quality andthe cell performance of HDR PECVD devices. By improving the cellperformance of the HDR PECVD cells, all of the benefits of the HDR PECVDare made available while reducing the drawbacks.

In particularly useful embodiments, a silicon based photovoltaic deviceis formed. After the formation of metal contacts (e.g., electrodes), theentire device is subjected to a low temperature anneal process. Theanneal process results in cell efficiency improvements of about 1-3%,which is a significant improvements in the solar industry. In accordancewith the present principles, competing benefits are weighed to achieve ahigh throughput manufacturing process while maintaining device qualityand performance.

It is to be understood that the present invention will be described interms of a given illustrative architecture having substrates andphotovoltaic stacks; however, other architectures, structures,substrates, materials and process features and steps may be variedwithin the scope of the present invention.

It will also be understood that when an element such as a layer, regionor substrate is referred to as being “on” or “over” another element, itcan be directly on the other element or intervening elements may also bepresent. In contrast, when an element is referred to as being “directlyon” or “directly over” another element, there are no interveningelements present. It will also be understood that when an element isreferred to as being “connected” or “coupled” to another element, it canbe directly connected or coupled to the other element or interveningelements may be present. In contrast, when an element is referred to asbeing “directly connected” or “directly coupled” to another element,there are no intervening elements present.

A design for a photovoltaic device may be created for integrated circuitintegration or may be combined with components on a printed circuitboard. The circuit or board may be embodied in a graphical computerprogramming language, and stored in a computer storage medium (such as adisk, tape, physical hard drive, or virtual hard drive such as in astorage access network). If the designer does not fabricate chips orphotovoltaic devices, the designer may transmit the resulting design byphysical means (e.g., by providing a copy of the storage medium storingthe design) or electronically (e.g., through the Internet), directly orindirectly.

Methods as described herein may be used in the fabrication ofphotovoltaic devices and/or integrated circuit chips with photovoltaicdevices. The resulting devices/chips can be distributed by thefabricator in raw wafer form (that is, as a single wafer that hasmultiple unpackaged devices/chips), as a bare die, or in a packagedform. In the latter case the device/chip is mounted in a single chippackage (such as a plastic carrier, with leads that are affixed to amotherboard or other higher level carrier) or in a multichip package(such as a ceramic carrier that has either or both surfaceinterconnections or buried interconnections). In any case, thedevices/chips are then integrated with other chips, discrete circuitelements, and/or other signal processing devices as part of either (a)an intermediate product, such as a motherboard, or (b) an end product.The end product can be any product that includes integrated circuitchips, ranging from toys, energy collectors, solar devices and otherapplications including computer products or devices having a display, akeyboard or other input device, and a central processor. Thephotovoltaic devices described herein are particularly useful for solarcells or panels employed to provide power to electronic devices, homes,buildings, vehicles, etc.

It should also be understood that material compounds will be describedin terms of listed elements, e.g., SiC or SiGe. These compounds includedifferent proportions of the elements within the compound, e.g., SiGeincludes Si_(x)Ge_(1-x) where x is less than or equal to 1, etc. Inaddition, other elements may be included in the compound and stillfunction in accordance with the present principles. The compounds withadditional elements may be referred to herein as alloys.

The present embodiments may be part of a photovoltaic device or circuit,and the circuits as described herein may be part of a design for anintegrated circuit chip, a solar cell, a light sensitive device, etc.The photovoltaic device may be a large scale device on the order of feetor meters in length and/or width, or may be a small scale device for usein calculators, solar powered lights, etc.

It is also to be understood that the present invention may includetandem (multi-junction) structures including sub-cells, which will bedescribed in terms of a particular material. Each sub-cell includes ap-doped layer, an n-doped layer and an undoped intrinsic layer.

Referring now to the drawings in which like numerals represent the sameor similar elements and initially to FIG. 1, an illustrativephotovoltaic structure 100 is illustratively depicted in accordance withone embodiment. The present embodiments may be part of a photovoltaicdevice or circuit, and the circuits as described herein may be part of adesign for an integrated circuit chip, a solar cell, a light sensitivedevice, etc. The photovoltaic devices described herein are preferably ona large scale on the order of feet or meters in length and/or width foruse in solar power. Hence, the present principles prefer large scaledevices with fabrication processes that provide higher throughputthrough faster deposition rates and easier processing (HDR PECVD). Thismay include rougher vacuum pressure in processing chambers, higherdeposition rates, etc.

Structure 100 includes a substrate 102 that permits a high transmittanceof light. The substrate 102 may include a transparent material, such asglass, quartz, silicon, a polymer, etc. or combinations thereof. A firstelectrode 104 includes a transparent conductive material. Electrode 104may include a transparent conductive oxide (TCO), such as, e.g., afluorine-doped tin oxide (SnO₂:F, or “FTO”), doped zinc oxide (e.g.,ZnO:Al), indium tin oxide (ITO) or other suitable materials. For thepresent example, a doped zinc oxide is illustratively employed forelectrode 104. The TCO 104 permits light to pass through to an activelight-absorbing material beneath and allows conduction to transportphoto-generated charge carriers away from that light-absorbing material.The TCO 104 may be deposited by a chemical vapor deposition (CVD)process, a low pressure CVD (LPCVD) process or a plasma-enhanced(PE-CVD) process.

A buffer layer 105 may be employed. Buffer layer 105 may includegermanium or silicon germanium and may include a p-type dopant such asB, Ga, In or combinations thereof. In one embodiment, the buffer layer105 includes a silicon-germanium alloy layer including germanium,silicon, optionally a p-type dopant, and hydrogen. In this embodiment,the atomic concentration of germanium may be greater than 50%. Dependingon the work-function of TCO 104, Si content in Ge can be varied. Thebuffer layer 105 has a greater atomic concentration of germanium than ap-doped layer 106 formed on the buffer layer 105.

The buffer layer 105 can be amorphous, microcrystalline, or singlecrystalline. The buffer layer 105 may include a hydrogenated material.For example, if the buffer layer 105 includes a hydrogenated amorphoussilicon germanium alloy, a hydrogenated microcrystallinesilicon-germanium alloy, a hydrogenated amorphous germanium, or ahydrogenated microcrystalline germanium, the hydrogenation of thematerial of the buffer layer 105 decreases localized electronic statesand increases the conductivity of the buffer layer 105. A hydrogenatedamorphous germanium may be formed using GeH₄ plasma. The buffer layer105 can be formed, for example, by the HDR PECVD process. In oneembodiment, the HDR PECVD process may include a power of between about100 mW/cm² and about 10 W/cm². The deposition temperature for eitherprocess may be maintained at between about 200 degrees C. to about 300degrees C., although other temperatures may be employed. A highdeposition rate (HDR) is provided that includes, e.g., 3-25angstroms/sec. The buffer layer 105 better transitions band gap energiesbetween layers.

The thickness of the buffer layer 105 can be from about 1 nm to about 10nm, although lesser and greater thicknesses can also be employed. Allthe barriers, e.g., between TCO 104 and the buffer layer 105 and betweenthe buffer layer 105 and the p+ layer 106, exist at valence bandlocations. One purpose of having the buffer layer 105 is to make amid-gap bridge between TCO 104 and p+ layer 106.

The buffer layer 105 better aligns the Fermi level between the TCO layer104 (e.g., ZnO) and the p-type layer 106 (e.g., a-SiC:H). The Fermilevel of the TCO layer 104 in the case of ZnO is about 4.5 eV. Withoutthe buffer layer 105, a larger Schottky barrier exists between the TCO104 and the p-type layer 106. With the buffer layer 105, the Fermilevels of TCO 104 and the buffer layer 105 are well-aligned reducing thebarrier offset. A conduction band between the buffer layer 105 and thep-type layer 106 is also well-aligned. In this way, the Schottky barrieris reduced or eliminated resulting in better conduction and moreefficient device operation by permitting higher open circuit voltage(V_(OC)) without sacrificing fill factor (FF). In particularly usefulembodiments, it is believed that increased crystallinity of the bufferlayer 105 correlates with increased open circuit voltage (V_(OC)) whileat least maintaining fill factor (FF).

The p-doped layer 106 may include an amorphous, microcrystalline, orsingle-crystalline p-doped silicon-containing material. The p-dopedlayer 106 can be a p-doped silicon layer including: silicon and at leastone p-type dopant and optionally hydrogen; a p-doped silicon-carbonalloy layer including: silicon, carbon, at least one p-type dopant andoptionally hydrogen. Other materials may also be employed.

In some cases, the p-doped layer 106 may include a hydrogenatedamorphous, microcrystalline, or single-crystalline p-dopedsilicon-containing material. A hydrogenated p-doped silicon-containingmaterial can be deposited in a process chamber having asilicon-containing reactant gas as a carrier gas. To facilitateincorporation of hydrogen in the hydrogenated p-doped silicon-containingmaterial, a carrier gas including hydrogen can be employed. Hydrogenatoms in the hydrogen gas within the carrier gas are incorporated intothe deposited material to form an amorphous or microcrystallinehydrogenated p-doped silicon-containing material of the p-doped layer106.

In one embodiment, p-doped or p-type layer 106 includes boron as adopant, although other p-type dopants may be employed. Layer 106 may bedeposited using diborane gas, trimethylborane or other gases. The p-typelayer 106 may have a thickness of between about 5-30 nm. In particularlyuseful embodiments, layer 106 includes amorphous SiC or Si. Theprocessing of the p-type layer 106 also can affect the efficiency of thedevice. For example, the deposition parameters for depositing the p-typelayer 106 may be selected to increase the effects of the buffer layer105.

The p-type layer 106 can be formed, for example, by the HDR PECVDprocess. As with the buffer layer 105 or any other layer in the device100, the HDR PECVD reduces costs by increasing throughput and reducingfabrication time. The HDR PECVD deposition may include a power ofbetween about 100 mW/cm² and about 10 W/cm². The deposition temperaturefor the process may be maintained at between about 200 degrees C. toabout 300 degrees C., although other temperatures may be employed. Thethickness of the p-type layer 106 can be from about 5 nm to about 30 nm,although lesser and greater thicknesses can also be employed.

In one embodiment, the p-type layer 106 (e.g., a-SiC:H) is aligned interms of the Fermi level with the TCO layer 104 (e.g., ZnO) or thebuffer layer 105 (e.g., Ge) (if employed). Processing continues with theformation of an intrinsic layer 110 formed on layer 106 from compatiblematerials. Intrinsic layer 110 may be undoped and may include anamorphous silicon material, e.g., hydrogenated amorphous Si (a-Si:H).The intrinsic layer 110 may include a thickness of between about 150-350nm, although other thicknesses are contemplated. The intrinsic layer 110may be formed using the HDR PECVD process, e.g., from silane gas andhydrogen gas. Using the HDR PECVD process for fabricating the intrinsiclayer 110 provides the greatest time savings due to the relatively largethickness of the intrinsic layer 110.

A doped layer 112 (e.g., an n-type layer) is formed on the intrinsiclayer 110. Layer 112 may include an n-type Si containing layer, e.g.,hydrogenated microcrystalline (μc-Si:H), single crystalline (Si) or anamorphous (a-Si) layer. Layer 112 may be deposited by the HDR PECVDprocess. The n-type layer 112 may have a thickness of between about 5-20nm. A back reflector and/or bottom electrode 114 may be formed using areflective metal, such as Ag, Al, etc. and may be combined with ZnO orother layers. A suitable metal deposition process may be employed, e.g.,physical or chemical vapor deposition, sputtering, evaporation, electroor electroless plating, etc. It should be understood that other materialselections, layers, structures, etc. may be employed in device 100including additional back reflectors, etc. or instead of backreflectors: tandem cells, etc.

It should also be understood that the structures depicted in FIG. 1 areillustrative and that other structures may be employed, e.g., the p-i-nstack can be reversed (n-i-p). In addition, one or more layers of thedevice 100 may be formed with the HDR PECVD deposition process, whileother layers may be formed using other processes.

The HDR PECVD deposition process is performed inside a vacuum chamber.The vacuum chamber employed here need not include ultrahigh vacuumpressures. Instead, in accordance with the present principles, a roughvacuum may be employed. The rough vacuum includes a pressure of betweenabout 1 mTorr and about 90 mTorr. With this vacuum and other parameters,high deposition rates can be achieved (e.g., 3-25 angstroms/second). Thecharacteristics of the HDR PECVD deposition process, e.g., thedeposition power, vacuum, processing temperatures, etc. result insignificant increases in device throughput.

It should be further understood that HDR PECVD may be employed todeposit amorphous Ge:H (a-Ge:H), amorphous Si:H (a-Si:H),microcrystalline Ge:H (μc-Ge:H), microcrystalline Si:H (μc-Si:H) for thep+, intrinsic, n+ and optional buffer layer. In addition, a-SiC:H andμc-SiC:H can be deposited by incorporating carbon gas for depositingthese layers. Combinations of these materials may also be useful, e.g.,a-SiGe:H, μc-SiGe:H may be deposited for the p+, intrinsic, n+ andoptional buffer layer. Other materials and//or additional elements(including appropriate dopants) may be incorporated with the layers asneeded or desired.

The HDR PECVD may be employed in forming all layers, one layer or anycombination of layers. The HDR PECVD process may be combined with otherprocesses (e.g., other deposition processes, wafer bonding, etc.)

In accordance with particularly useful embodiments, after the bottomelectrode or contact 114 is formed, the entire structure 100 undergoesan anneal process to improve the device performance. The anneal processmay include a rapid thermal anneal (RTA) and is targeted at a specifictemperature range (e.g., about 155 to about 250 degrees C.) for alimited duration (e.g., about 5 minutes to about 10 minutes). The annealis believed to cure defects and reduce contact resistance between metalelectrodes to semiconductor materials. In addition, an increase incrystallinity may be achieved especially at the top and bottom layersnearest the electrodes 104 and 114 (e.g., p-type layer 106 and n-typelayer 112).

Reduced defect zones 120 and 122 are disposed adjacent to the electrodesor contacts 104 and 114. The reduced defect zones 120 and 122 aredensified, more crystalline and more conductive than adjacent areas ofthe p-type layer 106 or n-type layer 112. In one embodiment, the defectzones 120 and 122 may extend into the intrinsic layer 110. Thephotovoltaic device 100 includes a panel of large length and width, butwith a small thickness. The anneal heats the conductive contacts 104 and114 and provides a temperature profile in the direction of the thicknessof the device. While temperature remains substantially uniform at thecontacts, the regions closest to the contacts 104 and 114 are annealedat the highest temperature. The temperature is reduced with depth intothe structure as the diode layer (p-i-n) are less thermally conductive.The temperature and duration of the anneal can be selected to determinean optimal performance of the device by controlling the reduced defectzones 120 and 122.

The reduced defect zones 120 and 122 may be configured (sized based ondepth through the thickness of the device 100) based on characteristicsof the photovoltaic device or cell 100. For example, the number oflayers, their dimension (e.g., thickness), their properties (e.g.,materials, formation process, etc.), and other considerations areemployed in determining the impact of the anneal on the device 100. Thereduced defect zones 120, 122 can be configured by selecting a durationand temperature for the anneal (between about 5 minutes and about 10minutes, at 155 to 250 degrees C.).

Referring to FIG. 2, a graph shows photovoltaic cell performance withoutan anneal in plot 202 and with an anneal in plot 204. The graph plotscurrent density (J) in mA/cm² versus voltage (V) in volts for the samedevice with and without the anneal after contact formation. As can beseen from the graph, the plot 204 is shifted to the right whichindicated improved device efficiency. The plots 202 and 204 correspondto a device that includes amorphous Si based layers processed using HDRPECVD processing. The device included p-i-n (p-type, intrinsic, n-type)layers. The anneal process was performed at 250 degrees C. for about 5minutes. In accordance with these findings, the HDR PECVD formationfollowed by the RTA results in improved device efficiency. Table I showsdata collected for the same cell with and without the anneal process.

TABLE I Open Circuit Short Circuit Type of Voltage (Voc) Current (Jsc)Efficiency processing Fill Factor (FF) (mV) (mA) (%) With anneal 70.42936.6 15.85 10.46 Without 66.15 876.9 14.8 8.59 anneal

In accordance with Table I, the overall efficiency was increased in thiscase by almost 2%. An increase in FF from 70.42 to 66.15 (6.5%) isachieved. Open circuit voltage (Voc) improves 6.8%, and short circuitcurrent improves 7%. Overall conversion efficiency improves by 21.7%.Cell performance may be optimized by anneal processes using specificannealing conditions. Advantageously, a fabricated device can be testedprior to anneal. Then, an appropriate anneal may be selected or testedto provide an optimized performance boost. This process can be performedwithout damaging the device.

In addition, large area processing refers to large scale devices havingan illustrative size on the order of feet or meters, e.g., 1.5 meters by0.6 meters. For large area deposition in manufacturing environments,devices may include solar panels. In accordance with the presentprinciples, a photovoltaic cell or panel made in particularly usefulembodiments has length and width dimensions of greater than about sixinches. Useful photovoltaic cells include an integrally formed radiationabsorbing area of greater than 0.25 square feet where the length andwidth of the cell may be varied. Low fabrication power is favored forsuch devices. Improvements as demonstrated in Table I result insignificant monetary savings as well as time and energy savings.

Referring to FIG. 3, methods for forming a photovoltaic device areillustratively shown. It should also be noted that, in some alternativeimplementations, the functions noted in the blocks may occur out of theorder noted in FIG. 3. For example, two blocks shown in succession may,in fact, be executed substantially concurrently, or the blocks maysometimes be executed in the reverse order, depending upon thefunctionality involved. It will also be noted that each block of theblock diagram and/or flowchart illustration, and combinations of blocksin the block diagram and/or flowchart illustration, can be implementedby special purpose hardware-based systems that perform the specifiedfunctions or acts, or combinations of special purpose hardware andcomputer instructions.

In block 302, one or more layers of a photovoltaic stack are depositedon a substrate by employing a high deposition rate plasma enhancedchemical vapor deposition (HDR PECVD) process. While it is possible toform these layers, e.g., a p-type layer, an optional buffer layer, anintrinsic layer, an n-type layer, etc. by different processes, it ispreferable to form these layers with the HDR PECVD process.

The HDR PECVD process preferably includes a deposition rate of 3-25angstroms per second or greater, and is performed in a vacuum chamberunder the following conditions: a vacuum pressure of between about 1mTorr and about 90 mTorr; a power of between about 100 mW/cm² and about10 W/cm² and a temperature between about 200 degrees C. to about 300degrees C. The buffer layer, if employed, is formed, in block 304,between a transparent electrode formed on the substrate and a p-typelayer.

In block 306, a contact or contacts are formed on the photovoltaic stackto provide a photovoltaic cell. One contact may be added when thephotovoltaic stack is completed, and the other contact is preferablyformed prior to formation of the photovoltaic stack, e.g., on thetransparent electrode of the substrate.

In block 310, an anneal process is performed on the photovoltaic cellafter the final contact is formed. The anneal is preferably performed ata temperature and duration configured to improve overall performance ofthe device.

In block 312, the anneal includes a duration controlled between about 5minutes and about 10 minutes. In block 314, the anneal includes atemperature range controlled between about 155 degrees Celsius and about250 degrees Celsius.

In block 316, the photovoltaic cell may be tested prior to the anneal todetermine the parameters needed for the anneal to provide a largestimprovement in operating performance.

In block 318, the photovoltaic cell is optimized. This may includeselecting parameters for the anneal in block 320. The anneal parameters(e.g., duration, temperature, etc.) are selected to optimize performanceof the photovoltaic cell based on characteristics of the photovoltaiccell. The characteristics of the photovoltaic cell may includematerials, dimensions, processes employed for formation, etc. Forexample, the thickness, materials, number of layers, etc. should beconsidered to ensure that a reduced defect zone is created adjacent tothe contacts. The reduced defect zone needs sufficient energy for theanneal to cure defects and reduce metal to semiconductor resistance. Thethermal energy needs to penetrate sufficiently but only for a limitedduration to ensure that better conductivity is achieved without damageto the photovoltaic stack. This may include thermal conductivityexperiments and/or computations for the device.

In block 322, processing continues as needed. For example, a backreflector or back reflectors may be formed. Other structures and layersare also envisioned.

Having described preferred embodiments for high efficiency solar cellsfabricated by inexpensive PECVD (which are intended to be illustrativeand not limiting), it is noted that modifications and variations can bemade by persons skilled in the art in light of the above teachings. Itis therefore to be understood that changes may be made in the particularembodiments disclosed which are within the scope of the invention asoutlined by the appended claims. Having thus described aspects of theinvention, with the details and particularity required by the patentlaws, what is claimed and desired protected by Letters Patent is setforth in the appended claims.

1. A method for forming a photovoltaic device, comprising: depositingone or more layers of a photovoltaic stack on a substrate by employing ahigh deposition rate plasma enhanced chemical vapor deposition (HDRPECVD) process; forming contacts on the photovoltaic stack to provide aphotovoltaic cell; and annealing the photovoltaic cell at a temperatureand duration configured to improve overall performance by increasing thecrystallinity of the photovoltaic stack in the regions of thephotovoltaic stack nearest to the contacts.
 2. The method as recited inclaim 1, wherein the one or more layers include a buffer layer between atransparent electrode formed on the substrate and a p-type layer of thephotovoltaic stack, wherein the buffer bridges an energy gap between thetransparent electrode formed on the substrate and the p-type layer ofthe photovoltaic stack.
 3. The method as recited in claim 1, wherein theHDR PECVD process includes a deposition rate of 3 angstroms per secondor greater.
 4. The method as recited in claim 1, wherein the HDR PECVDprocess includes a vacuum pressure of between about 1 mTorr and about 90mTorr.
 5. The method as recited in claim 1, wherein annealing includes aduration of between about 5 minutes and about 10 minutes.
 6. The methodas recited in claim 1, further comprising testing the photovoltaic cellprior to the annealing and applying the annealing to optimizeperformance of the photovoltaic cell based on characteristics of thephotovoltaic cell.
 7. The method as recited in claim 6, wherein thecharacteristics of the photovoltaic cell include materials, dimensionsand processes employed for formation.
 8. The method as recited in claim1, wherein the HDR PECVD process includes a power of between about 100mW/cm² and about 10 W/cm².
 9. The method as recited in claim 1, whereinthe temperature includes a range of between about 155 degrees Celsiusand about 250 degrees Celsius.
 10. The method as recited in claim 1,wherein the photovoltaic cell includes an integrally formed radiationabsorbing area of greater than 0.25 square feet.
 11. A method forforming a photovoltaic device, comprising: depositing a p-type layer ona transparent electrode of a transparent substrate; depositing anintrinsic layer on the p-type layer; depositing an n-type layer onintrinsic layer, wherein at least the p-type layer, the intrinsic layerand the n-type layer are deposited by employing a high deposition rateplasma enhanced chemical vapor deposition (HDR PECVD) process to form aphotovoltaic stack on the substrate; forming a contacts on the n-typelayer photovoltaic stack to provide a photovoltaic cell; and annealingthe photovoltaic cell at a temperature of between about 155 and 250degrees Celsius to improve overall performance by increasing thecrystallinity of the photovoltaic stack in the regions of thephotovoltaic stack nearest to the transparent electrode and the contacton the n-type layer.
 12. The method as recited in claim 11, wherein thephotovoltaic stack includes a buffer layer disposed between thetransparent electrode and the p-type layer, wherein the buffer layerbridges an energy gap between the transparent electrode formed on thesubstrate and the p-type layer of the photovoltaic stack.
 13. The methodas recited in claim 11, wherein the HDR PECVD process includes adeposition rate of 3 angstroms per second or greater.
 14. The method asrecited in claim 11, wherein the HDR PECVD process includes a vacuumpressure of between about 1 mTorr and about 90 mTorr.
 15. The method asrecited in claim 11, wherein rapid thermal annealing includes a durationof between about 5 minutes and about 10 minutes.
 16. The method asrecited in claim 11, further comprising testing the photovoltaic cellprior to the annealing and applying the annealing to optimizeperformance of the photovoltaic cell based on characteristics of thephotovoltaic cell.
 17. The method as recited in claim 16, wherein thecharacteristics of the photovoltaic cell include materials, dimensionsand processes employed for formation.
 18. The method as recited in claim11, wherein the HDR PECVD process includes a power of between about 100mW/cm² and about 10 W/cm².
 19. The method as recited in claim 11,wherein the photovoltaic cell includes an integrally formed radiationabsorbing area of greater than 0.25 square feet.
 20. A method forforming a photovoltaic device, comprising: depositing a buffer layer ona transparent electrode of a transparent substrate; depositing a p-typelayer on the buffer layer; depositing an intrinsic layer on the p-typelayer; depositing an n-type layer on the intrinsic layer, wherein atleast the p-type layer, the buffer layer, the intrinsic layer and then-type layer are deposited by employing a high deposition rate plasmaenhanced chemical vapor deposition (HDR PECVD) process to form aphotovoltaic stack on the substrate, the HDR PECVD process including adeposition rate of 3 angstroms per second or greater at a vacuumpressure of between about 1 mTorr and about 90 mTorr; forming a contacton the n-type layer photovoltaic stack to provide a photovoltaic cell;and annealing the photovoltaic cell at a temperature of between about155 and 250 degrees Celsius for a duration of between about 5 minutesand about 10 minutes to improve overall performance by increasing thecrystallinity of the photovoltaic stack in the regions of thephotovoltaic stack nearest to the transparent electrode and the contacton the n-type layer.
 21. The method as recited in claim 20, furthercomprising testing the photovoltaic cell prior to the annealing andapplying the annealing to optimize performance of the photovoltaic cellbased on characteristics of the photovoltaic cell.
 22. The method asrecited in claim 21, wherein the characteristics of the photovoltaiccell include materials, dimensions and processes employed for formation.23. The method as recited in claim 20, wherein the HDR PECVD processincludes a power of between about 100 mW/cm² and about 10 W/cm².
 24. Themethod as recited in claim 20, wherein the photovoltaic cell includes anintegrally formed radiation absorbing area of greater than 0.25 squarefeet.